Long range transfer of positive charge between dopant molecules in a rigid glassy matrix

Abstract

Pulse radiolysis has been used to observe and measure the kinetics for intermolecular positive charge (hole) transfer from biphenyl⁺ or pyrene⁺ ions to TMPD molecules in rigid 2‐chlorobutane glass at 77 K. These hole transfers occur over distances of about 17 Å at 10⁻⁶ s, increasing to about 34 Å at 10² s. The kinetic data are interpreted in terms of current theories which treat electron transfer processes as radiationless transitions. Estimates of the required electron exchange interactions based on the usual electron tunneling models can not explain the fast reactions observed, even when Coulombic effects on the ’’barrier’’ are considered. A superexchange model is proposed which involves interactions propagated by both negative and positive ion states of the solvent. This model adequately interprets the data in terms of a dominant role of the solvent positive ion states, and is also applicable to negative charge transfer in the condensed phase. In samples containing only one solute (biphenyl or pyrene), ion recombination with Cl⁻ removed about one third of the positive ions of the solute between 10⁻⁶ and 10² s. The data on intermolecular hole transfer between two solutes must be corrected for the effects of ion recombination. The correction is simple and quantitative only when it is possible to observe decay of the reactants without substantial spectral overlap from the products. Because the effects of ion recombination on product growths are complex, it is not presently possible to use the growths to measure reliably the kinetics intermolecular positive charge transfer.